1. Field of the Invention
This invention relates generally to a power amplifier circuit employing a technique for maintaining the amplifier's operating point constant over time and, more particularly, to a power amplifier circuit employing a current sensing resistor that is switched into the power amplifier circuit when the amplifier circuit is in a quiescent state to measure and maintain the small signal gain and linearity of the amplifier constant over time.
2. Discussion of the Related Art
Communications systems that transmit RF signals carrying information typically employ power amplifiers to amplify the transmit signal so that it has enough power to be received and deciphered by a receiver at a distant location. Digital communication systems of this type typically have highly complex and highly precise modulation waveforms in order to maximize utilization and revenue from precisely assigned radio frequency band allocations. Thus, the specifications of a digital communications system and its associated components usually are very strict. For example, precise spectral control must be maintained in the system over varying operating conditions and environments and over the system life.
Power amplifiers are among the most critical components of a digital communications system because they directly affect the linearity, distortion and spectral control of the transmit signal. Nonlinearities within the power amplifier introduce signal harmonics, inter-modulation products and other distortions and spurious counterparts of the RF signal being transmitted that cause interfering signals to other frequency channels. Additionally, the power amplifiers must maintain a constant small signal gain (SSG) despite the fluctuations in loading, drive and device aging that may act on the amplifier. In particular, for the commonly employed laterally diffused metal oxide semiconductor (LDMOS) transistor amplifier, the SSG will vary as the quiescent drain current of the amplifier changes due to amplifier aging and temperature changes.
Because it is necessary that the SSG performance of a power amplifier remains constant over the transmitter circuit lifetime, which may be years, various techniques are known in the art to ensure that the SSG remains constant. For example, because most of the drift in the quiescent drain current of the amplifier occurs during the initial 10-20 hours of operation, it is known in the art to set the quiescent drain current of the amplifier at a value higher than the desired operation point of the amplifier, and then allow the quiescent current to drift into the nominal operating condition after several hours of operation. However, not only will the power amplifier circuit operate outside of its desired operating point at the beginning of the amplifier's life when the drift in the quiescent current is greatest, the amplifier quiescent current will also continue to drift past the desired operating point towards the end of its useful life. This results in a less than optimum performance at the beginning and towards the end of the amplifier's life.
It has heretofore also been known to “bum in” an amplifier at the beginning to allow the quiescent drain current of the amplifier to stabilize prior to setting the operating point of the amplifier. However, this is an expensive alternative in a high volume production environment because many amplifiers are required to be burned in for several hours, significantly increasing cost of production. Also, the “bum in” technique does not affect the drift of the SSG towards the end of the amplifier's useful life.
Some amplifier circuits employ active DC bias compensation to maintain the SSG constant throughout the life of the amplifier. For example, some power amplifier circuits incorporate a low value sensor resistor in series with the drain or collector supply of the output of the transistor amplifier. The resistor is typically part of a current mirror within a feedback loop in the amplifier circuit that sets the gate voltage of the transistor amplifier, or base current for bipolar applications, which sets the quiescent drain current of the transistor amplifier. Alternatively, the amplifier circuit may employ an analog-to-digital (A/D) converter to measure the voltage across the sensor resistor, and a microcontroller to set the gate voltage, and hence the quiescent drain current of the transistor amplifier.
Due to the small value of the sensor resistor, these amplifier circuits employing an active DC bias typically have poor accuracy. A larger sensor resistor improves accuracy, but degrades the efficiency of the amplifier circuit during operation due to the power dissipated in the sensor resistor. In the case of the more elaborate approach of using A/D converters and microcontrollers, the cost of monitoring, measuring and controlling the quiescent drain current while the power amplifier is in operation is difficult and expensive. In all cases, the sensor resistor reduces the maximum output power of the amplifier, thereby degrading the linearity of the amplifier.